The invention disclosed and claimed herein generally pertains to an improved inversion recovery (IR) method of magnetic resonance (MR) imaging. More particularly, the invention pertains to a method of the above type which retains the intrinsic phase information from the originally acquired data in reconstructing an image. Even more particularly, the invention pertains to a method of the above type which preserves the directional information of the magnetization vectors, while effectively reducing noise and artifacts.
As is well known by those skilled in the art, an inversion recovery-prepared MR pulse sequence includes a 180.degree. RF inversion pulse, followed by a 90.degree. RF excitation pulse after an inversion recovery time (TI). It has long been recognized that such sequences offer potentially superior T1-contrast because the range of the longitudinal magnetization is doubled by the 180.degree. inversion pulse. However, the most successful application of the IR sequences so far is still restricted to suppression of species with certain T1. The comparatively rare use of the IR sequence for T1-contrast enhancement is mainly due to two significant disadvantages. First, IR sequence time is usually significantly longer than other imaging sequences because of the long inversion recovery time, and the total imaging time may become prohibitive for multi-slice imaging. Secondly, IR images acquired at certain inversion times may display a reversed contrast, so that tissues of longer T1's appear brighter than tissues of shorter T1's. This anomalous phenomenon is known to originate from the widely-used magnitude reconstruction on commercial MRI scanners, and has been reported to cause confusion and difficulty in image interpretation.
The 2D Fourier Transform of the time-domain data acquired in a spin echo or fast spin echo inversion recovery sequence can be, in general, expressed as:
S(x,y)=I(x,y)e.sup.j(.phi..sup..sub.i .sup.(x,y)+.phi..sup..sub.e .sup.(x,y)) Eqn.(1)
In Equation (1), .phi..sub.i (x,y) is the intrinsic phase, determined by the sign of the spin magenetization at the time of the excitation. .phi..sub.i (x,y) can therefore only take a value of either 0 or .pi.. .phi..sub.e (x,y) is a ubiquitous phase error term, which is independent of time, but generally varies with spatial locations. Possible sources of contribution to .phi..sub.e (x,y) include complex passband RF receiver filters, mis-centering of the data acquisition windows, poor gradient compensation, B.sub.o -field inhomogeneity, and phase shifts due to RF receiver coils In a conventional reconstruction, however, only magnitude is typically used. As a result, both the intrinsic phase .phi..sub.i (x,y) and the phase error term .phi..sub.e (x,y) have, in the past, generally been discarded. I(x,y) is the magnitude of the image vector, as obtained in a conventional magnitude reconstruction. I(x,y) is generally a function of the initial magnetization M.sub.o (x,y), as well as some imaging and tissue parameters , and can be expressed as follows: EQU I(x,y)=.vertline.M.sub.o (x,y)[1-2e.sup.(-TI/T1) -e.sup.(-TR/T1) +2e.sup.((-TR-TE/2)/T1) ].vertline. Eqn. (2)